This invention relates to liquid-crystal displays and, more particularly, to a display employing a bistable liquid-crystal medium such as a ferroelectric material.
Liquid-crystal displays are employed frequently in numerous situations for the presentation of both alphanumeric data and pictorial data. The image presented on the display is composed of an array of pixels disposed in a matrix of rows and columns. In the typical construction of a liquid-crystal display, a layer of nematic liquid-crystal material is disposed between two layers of electrode structure. One of the electrode structures, the top electrode structure by way of example, is formed as a set of column conductors and the other electrode structure, namely the bottom electrode structure, is formed as a set of row conductors.
A characteristic of a display formed of twisted nematic or super-twisted nematic liquid-crystal material is the need to continuously repeat excitation of each pixel. Each pixel is formed at the intersection of a row conductor and a column conductor by the development of an electric field between the row conductor and the column conductor. The electric field alters the state of the liquid-crystal material to impart rotation of an electric vector of light propagating through the liquid-crystal material. The light propagates in a direction perpendicular to a plane of an electrode structure. It is the practice to employ alternating voltage to excite the electrode structures so as to avoid an electrochemical reaction between the electrode structures and the liquid crystal material.
In the presence of an applied electric field, the nematic liquid-crystal material undergoes the aforementioned change in state to impart the rotation to the electric vector. However, upon release of the applied electric field, the nematic liquid-crystal material returns to its original state thereby terminating the rotation of the electric vector of the light. Therefore, with nematic liquid-crystal displays, it is the practice to continuously retransmit electrical signals along the electrodes of the top and the bottom of electrode structures to refresh the displayed image at sufficient frequency to provide a person viewing the image with an image that appears to be present continuously.
Another form of liquid-crystal display employs a bistable material such as a ferroelectric-crystal material. Until recently, such displays found little use because the liquid-crystal material is operative only at elevated temperatures, such as 70 degrees centigrade. However, there is available now a ferroelectric liquid-crystal material which is operative at room temperature. Therefore, such displays could be employed in the numerous situations wherein nematic liquid-crystal displays are presently employed.
A problem arises in that presently available electronic systems for activating liquid-crystal displays do not take advantage of the bistable characteristic of ferroelectric liquid-crystal material. In particular, it is noted that the bistable characteristic allows the display to be operated without the need for repetitive refreshing of the image. Rather, a single pulse of electric field of sufficient strength is adequate to permanently alter the state of the liquid-crystal material, the state being maintained until an electric field of opposite sense is applied to restore the original state of the liquid-crystal material. Thus, a single pulse of electric field suffices to induce a rotation of the electric field vector of light propagating through the display at the site of a pixel; the pixel maintains its state of illumination until such time as a pulse of electric field of the reverse sense is applied to the ferroelectric liquid-crystal material by the electrode structures. The freedom from the need of continuous refreshing of the display, provided by the bistable liquid-crystal material, should allow for simplification of electric drive circuitry, as well as the capacity to drive significantly larger displays than has been done heretofore.